This invention generally relates to concentrator circuits, and more specifically to wide band, low loss, low distortion electronic concentrator circuits.
Telephone switching networks incorporate a wide variety of electrical switching elements in diverse applications. One such application is a front end concentrator. A front end concentrator connects to a plurality of telephone lines and selectively connects one of those telephone lines to a common line or signal path, such as a shared line circuit, under the control of selection signals. When a selection is made, signals pass from the selected telephone line to the common signal path and from the common signal path to the selected telephone line as if the common signal path were dedicated to the telephone line.
The switching elements that constitute the concentrator must comprise bilateral, wide band, low loss, low distortion switching elements. Conventionally, these switching elements comprise electromechanical relays and related circuits. When the contacts on a relay close, the resulting metal-to-metal connection forms a conductive path that produces essentially no loss of, and no distortion to, any signal passing through the contacts. The connection allows bilateral signal transmission over a wide range of frequencies. The contacts can withstand voltage and current levels that are dependent only on the rating of the contacts themselves. These relays also provide lightning and other transient signal protection to control circuits because the air gap between the contacts and the coil inherently isolates transient signals on the conductive path from the control circuitry that connects to the coil.
Unfortunately, such electromechanical devices have a number of known characteristics that are detrimental in many applications including telephony concentrator applications. Life expectancy of a relay depends upon contact wear. As contact wear is not predictable on an accurate basis, it is difficult to predict relay life. Moreover, electromechanical relays tend to be expensive, and this expense is subject to high cost multiples in concentrators for telephone switching networks where one such relay can be used for each of the many telephone lines connected to a single concentrator and where the telephone switching network may include many concentrators. Electromechanical relays also increase other manufacturing costs as they are discrete elements and must be mounted on circuit boards individually.
Special switching circuits have been proprosed from time to time to replace the electromechanical relay. Reversely poled, silicon-controlled rectifier and other transistor circuits have been suggested. However, in telephone concentrator applications both dc hook signals and analog voice signals pass through such a switching circuit and the reversely poled transistor and SCR circuits introduce a crossover distortion as these circuits tend to stop conducting near the zero voltage, or current crossover, point. Circuitry can be added to compensate for such distortion. However, this and other circuitry that may be necessary to provide proper operation, increase the overall costs of manufacture for the concentrator.
More recently, a gated diode switch has been proposed that can be manufactured with large scale integration manufacturing techniques. The large number of switches in concentrators makes the use of large scale integration manufacturing techniques highly desirable, especially in view of the attendant cost savings that can be achieved. The proposed switch, however, has certain disadvantages. First, this circuit requires high voltage differences in the order of up to 600 volts. When the switch is located in the standard tip and ring conductors to or from a subscriber's handset, certain precautions must be taken to protect customers and telephone company personnel against inadvertent contact with these voltages. This switching circuit also requires three separate voltages and this, of course, introduces complexities in the power supplies that serve the telephone switching network. Moreover, it would appear from available information that the proposed switch may require compensation for crossover distortion when the switching circuit conveys analog voice signals.